With a recent increasing demand for lightweight or higher output of automobiles for the purpose of a decrease in exhaust gas or improvement of fuel efficiency, high stress design has also been required for valve springs, clutch springs, suspension springs and the like which are used in engines, clutches, suspensions and the like. These springs tend to have higher strength and thinner diameter, and the load stress tends to further increase. In order to comply with such a tendency, a spring steel having higher performance in fatigue resistance and settling resistance has been strongly desired.
Further, in order to realize lightweight while maintaining fatigue resistance and settling resistance, hollow pipe-shaped steel materials having no welded part (that is to say, seamless pipes) have come to be used as materials of springs, instead of rod-shaped wire rods which have hitherto been used as materials of springs (that is to say, solid wire rods).
Techniques for producing the hollow seamless pipes as described above have also hitherto been variously proposed. For example, Patent Document 1 proposes a technique of performing piercing by using a Mannesmann piercer which should be said to be a representative of piercing rolling mills (Mannesmann piercing), then, performing mandrel mill rolling (draw rolling) under cold conditions, further, performing reheating under conditions of 820 to 940° C. and 10 to 30 minutes, and thereafter, performing finish rolling.
On the other hand, Patent Document 2 proposes a technique of performing hydrostatic extrusion under hot conditions to form a hollow seamless pipe, and thereafter, performing spheroidizing annealing, followed by performing extension (draw benching) by Pilger mill rolling, drawing or the like under cold conditions, resulting in the improvement of productivity and quality. Further, in this technique, it is also shown that annealing is finally performed at a predetermined temperature.
In the respective techniques as described above, when the Mannesmann piercing or the hot hydrostatic extrusion is performed, it is necessary to heat at 1,050° C. or more or to perform annealing before or after cold working, and there is a problem that decarburization is liable to occur in an inner peripheral surface and outer peripheral surface of the hollow seamless pipe during processing under hot conditions or working or in a subsequent heat treatment process. Further, at the time of cooling after the heat treatment, decarburization (ferrite decarburization) caused by the difference between the solute amount of carbon in ferrite and that in austenite also occurs in some cases.
Occurrence of the decarburization as mentioned above brings about a situation that surface layer parts of the outer peripheral surface and inner peripheral surface are not sufficiently hardened during quenching in the production of springs, which causes a problem that it becomes impossible to ensure sufficient fatigue strength in springs to be formed. In addition, when there are flaws therein, the flaws become points on which stresses converge, and constitute a factor of early fractures thereof.
In addition, enhancement of fatigue strength in the case of general springs has generally been performed by applying residual stress to the outer surfaces of the springs by means of shot peening or the like. In the case of springs formed from a hollow seamless pipe, shot peening or the like cannot be given to the inner peripheral surfaces of the springs, and besides, traditional working methods are liable to bring about flaws in the inner peripheral surface. Thus, it is necessary to strictly control qualities with regard to decarburization, flaws and the like as compared with the case of solid materials.
As a technique for solving the above-described problems, a technique disclosed in Patent Document 3 is also proposed. In this technique, a rod material is hot-rolled, followed by piecing with a gun drill, and being subjected to cold working (draw benching or rolling), thereby producing a seamless steel pipe. Accordingly, heating can be avoided during piercing or extrusion.